Historically, when it was necessary to completely remove a disc from between adjacent vertebrae, the conventional procedure was to fuse the adjacent vertebrae together creating a situation where no motion is present between the spinal segments. This “spinal fusion” procedure, which is still in use today, is a widely accepted surgical treatment for symptomatic lumbar and cervical degenerative disc disease.
More recently, there have been developments in the field of disc replacement, namely disc arthoplasty, which involves the insertion of an artificial intervertebral disc implant into the intervertebral space between adjacent vertebrae. Such a disc implant allows limited universal movement of the adjacent vertebrae with respect to each other. The aim of total disc replacement is to remove pain generation (caused by a degenerated disc), restore anatomy (disc height), and maintain mobility in the functional spinal unit so that the spine remains in an adapted sagittal balance. In contrast with fusion techniques, total disc replacement generally preserves mobility in the motion segment and mimics physiologic conditions.
One such intervertebral implant includes an upper part that can communicate with an adjacent vertebrae, a lower part that can communicate with an adjacent vertebrae, and an insert located between these two parts. To provide an anchor to the adjacent vertebrae, each part includes a vertically extending keel. Examples of this type of implant are disclosed in U.S. Pat. No. 5,314,477 (Marnay), U.S. Pat. No. 6,936,071 (Marnay et al.), and U.S. Pat. No. 7,204,852 (Marnay et al.), which are hereby incorporated by reference.
It will also be noted that in order to provide a keel slot in a vertebra, a cutting of the bone needs to be performed. Typically the cut is made by chiseling, drilling or milling. Combinations of these procedures are possible too. Exemplary of such prior art devices and methods are those disclosed in USPA 2004-0215198 (Marnay et al.) and USPA 2006-0064100 Bertagnoli et al.), which are hereby incorporated by reference.